The correct answer is: B. are less susceptible to failure.
Earthen dams are less susceptible to failure than gravity dams because they are more flexible and can withstand earthquakes and other natural disasters better. They are also less expensive to build and maintain. However, they require more land and can be more difficult to construct.
Gravity dams are made of concrete or stone and are designed to withstand the force of water by their own weight. They are very strong and can be built very tall, but they are also very expensive to build and maintain. They are also more susceptible to failure than earthen dams, especially in areas with earthquakes or other natural disasters.
Option A is incorrect because earthen dams are less expensive to build than gravity dams.
Option C is incorrect because earthen dams do not require sound rock foundations. They can be built on a variety of foundations, including sand, clay, and gravel.
Option D is incorrect because earthen dams require more skilled labor to build than gravity dams.